execute是我们提交一个线程的时候，线程池执行的我们运行线程的一个api
ThreadPool 有几个比较重要的参数会涉及到这个源码的操作
- corePoolSize：核心线程数；
- maxMumPoolSize:最大线程数
- BlockingQueue:阻塞队列
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
// 获得ctl参数；其中高3位表示线程的状态。后面29位表示线程的数量
int c = ctl.get();
// 如果线程数少于核心线程数，就创建一个worker（本质也是一个线程，执行我们的传入的线程）,
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
// 这里校验isRunning.. 如果我们关闭线程池的时候，这里是需要做判断的
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
// 添加一个worker。firstTask 是我们传进来需要执行的线程，core：是为了后面判断的时候用coolPoolSize还是 maximumPoolSize
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
// 这里是异常情况，如果是线程池shoutdown了，就直接退出了
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
// 获得线程数
int wc = workerCountOf(c);
// 这里判断是不是大于了最大的线程数，如果我们是第一个线程，核心线程>1，不会走这个判断，
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// 少于核心线程数，把工作线程数+1
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
// 新建一个wokrer
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
// 这里和上面的校验一样，做了双层校验
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
// 新增一个wokrer到HashSet集合中
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
// 如果添加成功的话，就让worker里面的Thread启动执行我们传进去的线程代码
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
那么我们的线程里面run方法里面的代码在哪执行，然后后面又是怎么从队列里面拿出下一个任务出来执行的呢？
所有的秘密都在wokrer中构造方法中
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
// 这里会创建一个线程（将当前worker，因为wokrer实现了Runnable方法）
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
runWorker是真正将线程跑起来的方法
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
// 如果是第一个线程的话，task不为null。那么就执行下面的代码
// 这是一个循环。当我们第一个任务执行完后，task会被赋值为null，然后就开始从队列面拿任务执行，如果task不为null的话就继续执行任务，不断循环直到task为null（队列为空）
while (task != null || (task = getTask()) != null) {
w.lock();
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
// 调用我们传入的线程任务
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
// 如果队列为空，这种情况就会跑到这里
processWorkerExit(w, completedAbruptly);
}
}
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
// 如果线程数大于核心线程数，或者allowCoreThreadTimeOut 为true 这里才为true
// allowCoreThreadTimeOut 默认是false。如果为false的话，则会常驻核心线程
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
// 如果是true的话，就执行从队列里面poll，当前线程会等待指定的keepAliveTime时间，如果超时则返回为null。
// 否则的话就是take（生产者，消费者模型，如果有任务了就执行，没有任务就阻塞.. 这样就保证了核心线程数是不会销毁的）
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
线程池如何保证核心线程数。
队里为空的时候，task就会返回null。runWorker就会跳出循环执行processWorkerExit方法
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// 这里会移除一个worker
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
// 这里重新将wokrer加进去，保证核心线程数不变
addWorker(null, false);
}
}